Method for extending the growth of vapor-liquid-solid grown crystals

ABSTRACT

By maintaining the desired temperature gradient in the molten solvent as the crystal grows and by replenishing the solvent lost by segregation into the growing crystal, a continuous filament crystal is formed. As the crystal is formed it is pulled from the formation area at its speed of growth. A vaporous compound containing the solvent element furnishes a vapor containing the solvent. The solvent vapor condenses on the molten solvent to replenish the solvent lost by segregation into the growing crystal.

United States Patent Inventor Raymond W. Conrad Fayetteville, Tenn.

Appl No. 821,729

Filed May 5, 1969 Patented Sept. 21, 1971 Assignee The United States of America as represented by the Secretary of the Army METHOD FOR EXTENDING THE GROWTH OF VAPOR-LlQUID-SOLID GROWN CRYSTALS 9 Claims, 2 Drawing Figs.

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[56] References Cited UNITED STATES PATENTS 3,053,635 9/1962 Shockley 23/208 A 3,232,745 2/1966 Rummelet al 23/2235 Primary Examiner- Earl C. Thomas Assistant ExaminerHoke S. Miller Attorneys-Harry M. Saragovitz, Edward J. Kelly, Herbert Berl and Aubrey .I. Dunn o o o o o o o o o o o 6 I is oooo ooloooooz ll I F|G.l 8 i TEMPERATURE PROFILE SUBSTRATE POSlTiON PATENTEUSEP21 lsm N T zOrEwOa mJEOmm mmpzmumimk -ooooo o oooo v w Q Raymond W. Conrad,

INVENTOR METHOD FOR EXTENDING THE GROWTH OF VAPOR- LIQUID-SOLID GROWN CRYSTALS BACKGROUND OF THE INVENTION This invention relates to single crystals of elements and compounds, and in particular, to those which are difficult to prepare by conventional crystal-pulling techniques.

In 1964, Wagner and Ellis reported in Applied Physics Letters,volume 4, page 89, the growth of small whiskerlike crystals of silicon by a process now commonly known as the vapor-liquid-solid (v-l-s) technique. Since then, several other substances have been grown in small, single crystal form by the v-l-s technique; for example, silicon carbide in 1967 by C. E. Ryan, et al. as reported in the Journal of Crystal Growth" volume 1, page 225.

These is a need for single filament crystals in the manufacture of composite structures. In the past, composite structures have been made using the small crystals available from the v-ls technique. The crystals are interspersed throughout the metal to obtain a desired characteristic such as increased strength, e.g., aluminum oxide crystals within nickel.

The longer the crystal, the more strength is adds to the composite. This invention provides a method for growing crystal filaments of any desired length. By incorporating longer filaments into composite structures, the strength of the composite is greatly increased over what it would be had the smaller crystals been used.

In the v-l-s technique there are three distinct phases. The vapor phase is composed of substances which can be thermally decomposed, reduced, or otherwise broken down to produce an element of the desired substance. In practice, an inert gas is generally added to the vapors for improved control of vapor concentrations and flow rates. For example, if growing silicon crystals, then silane in one choice for the vapor phase. Silane is thermally decomposed to silicon and hydrogen at moderate temperatures. Another choice is silicon tetrachloride and hydrogen. At elevated temperatures, silicon tetrachloride is reduced by hydrogen to produce silicon and hydrogen chloride.

lf, instead of silicon, the desired crystal was silicon carbide, then a silane and methane mixture is used and thermally decomposed to produce silicon, carbon and hydrogen. Trimethylchlorosilane alone is also a suitable vapor.

The liquid phase must be a solvent for the desired substance. However, the solubility must not be infinite. A low segregation coefficient and a positive temperature coefiicient are desirable. In general, the desired substance and the solvent should have a phase diagram of the simple eutectic type. The solvent should be thermally stable at the elevated temperatures employed, should have a low volatility at these temperatures, and should not chemically react with any of the vapor species.

The solid phase is, of course, the desired crystalline substances. This phase may either originate spontaneously by nucleation or crystallization at the vapor-liquid or liquid-inert substrate interfaces, or, a single crystal seed of the desired substance may be used as the substrate.

During a v-l-s crystal growth, the following chemical and physical processes occur: The gaseous reactants are decomposed to the element or elements comprising the desired crystal and the resulting vapors impinge on the surface of a molten solvent drop. A solvent drop is supported on either an inert substrate or a seed crystal by conventional means such as disclosed in U.S. Pat. No. 3,232,745 to Rummel et al. The vapors of the desired element or elements condense preferentially on the solvent drip and thereby saturate it. Because of the continuous supply of vapors, the surface of the drop supersaturates, and a concentration gradient is established normal to and inward from the vapor-liquid interface. A precipitation of the desired crystalline substance then occurs preferentially at the solid-liquid interface. This precipitation is epitaxial on the seed crystal or the spontaneously nucleated crystal, whichever the case may be. The net result is generally an elongated crystallite with a solvent droplet terminus. (In certain instances, such as when the solvent wets the solids substrate, large area crystal may be produced.)

Crystal growth is terminated by one or both of the following processes: (1) The growing crystal pushes the solvent drop into a region of the reactor such that a'negative temperature gradient from the liquid-vapor to the liquid-solid interface is produced. This would decrease the solubility of the desired element or elements at the surface of the drop relative to the interior and either simply terminate the dissolution process or induce spurious nucleation. (2) The solvent is depleted because of its solid solubility in the precipitating crystal. If both of the above conditions can be eliminated then the limitations on the ultimate size of the crystal can be removed. The object of this invention is the removal of these restrictions, thus permitting growth of large area crystals of reasonable size or growth of very long, small diameter, filamentary crystals.

SUMMARY OF THE INVENTION This invention provides a method for growing a continuous, single crystal filament of a large variety of substances. For systems in which direction v-l-s mechanism is operative, continuous filament growth can be achieved by pulling a filament with a molten solution drop on the end from a heated zone of vaporous reactants. By pulling the filament from the heated zone of vaporous reactants at the speed of filament formation, the molten drop is maintained correctly in the heated zone and the desired temperature gradient is maintained across the molten drop. Continuous single crystal growth results.

For a growth to be continuous, the molten drop must be maintained. Ideally, the solvent has a very small segregation coefficient in the compound forming the filament. Even so, some provision for replenishing the solvent must be made if growth in to be continuous. By incorporating a suitable volatile, thermally decomposable (or reducible) compound containing the solvent element(s) in the vapor, the solvent may be continuously replenished. Classical condensation theory shows that the vaporous solvent preferentially condenses on the molten drop rather than on the sides of the growing filament.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of the apparatus for achieving the crystal growth as herein described; and

FIG. 2 is the temperature profile of reactor tube 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention will be described with reference to the for mation of a single crystal filament of silicon carbide. It is to be understood that the same principles apply to large area crystal growth.

Referring to the drawing, FIG. 1 shows a device wherein the filament growth is obtained. Reactor tube 2 has a temperature profile as shown in FIG. 2 because of heating coils 16. It is desired that molten drop 4 have a temperature gradient as projected from FIG. 2 to FIG. I. The filament or crystal will form on the side of the molten drop that is the lowest in temperature. The filament starts forming a substrate l2. As the filament forms, it is withdrawn. from the reactor tube. It is desired that the filament be extracted at the same speed as it is being formed. In this manner, molten solvent drop 4 remains in the same position within reactor tube 2. The necessary reactants are supplied through entrance 6 and exit through exhaust 8. Substrate holder 10 secures substrate 12. Guides 14 maintain the substrate in the center of the reactor tube and, after the substrate has been withdrawn, maintain the filament in the center of the reactor tube. Heating coils l6 maintain the desired temperature gradient shown in FIG. 2.

For silicon carbide, several suitable solvents are available. lron, silicon, gold, chromium, and rhenium have been successfully used. Gallium, indium and tin are other possible solvents. Using iron as an example, a filamentary seed crystal 18 of silicon carbide is coated on its tip with iron, e.g., by evaporation. This seed crystal is affixed to substrate 12. The substrate is mounted in substrate holder 10, so that the growing crystal may be withdrawn from the heated portion of the reactor tube as the filament lengthens. This is very important since the solvent drop must remain in a constant position in the reactor as the crystal lengthens. The entire reactor may be moved instead of the substrate holder to achieve the same effect.

In this particular example, the reactor system is heated to the operating temperature (roughly the melting point of the particular solvent used--for iron, approximately l,550 C), and the vapor flow started. The withdrawal rate of the crystal is adjusted to match the growth rate which is a function of temperature, vapor composition, reactor geometry, etc. To insure that crystal growth will not terminate by the second mechanism described earlier, a volatile decomposable or thermally decomposable compound of the solvent is introduced into the vapor flow. The vaporous solvent element or elements will condense on the liquid solvent. The volatile decomposable or thermally decomposable compound of the solvent could be iron pentacarbonyl. This compound is a volatile liquid at room temperature and decomposes into iron and carbon monoxide at temperature greater than 200 C. Undoubtedly other such compounds exist also.

The concentration and flow rate of the iron pentacarbonyl is adjusted such that the rate of disappearance of iron by segregation is exactly matched by the rate of condensation. Now, with both growth termination mechanisms overcome, growth is maintained as long as reactants are supplied.

Crystals of some materials grow by a two-step process. The first step is a very rapid v-l-s growth of a very thin leader" crystal at rates of several cm./sec. The second step is lateral thickening of the leader by simple vapor deposition. ln systems exhibiting this behavior, the leader crystal is pulled from the reaction zone at its characteristic propagation velocity, and thickened to a usable size in a long, vapor deposition section downstream. The process is preferably conducted in two stages since the optimum vapor composition for the two steps is not necessarily the same.

A unique method for the growth of extended crystals employing vapor-liquid-solid techniques has been described. The present invention provides for the growth of either large or small area crystals depending on the choice of solvent substrate and operating conditions. The limitations on crystal size inherent in conventional vapor-liquid-solid techniques are overcome by this invention.

I claim: 1. A process for growing a single crystal filament comprising the steps of continuously decomposing a compound in a heated environment to obtain a continuous vapor having the elementor elements of the crystal to be grown, said element or elements being selected from the group consisting of silicon and silicon carbide; causing said element or elements to condense on a molten solvent drop that has been placed on a substrate and in said heated environment; supplying a vapor containing said molten solvent to the environment around said molten drop to condense on said molten drop to maintain the quantity of said molten drop substantially constant; and maintaining a substantially constant predetermined temperature gradient in said molten drop to maintain said drop in a molten state and to cause said element or elements to crystalize on said substrate and grow as a continuous filament crystal at a cooler portion of said molten drop. 2. A process as set forth in claim 1 wherein said temperature is maintained by translating said continuous filament crystal in a direction opposite said continuous filament crystal growth and at the rate of growth of said continuous filament crystal to thereby cause said molten drop to maintain a predetermined temperature gradient therein.

3. A process for growing a single crystal filament as set in claim 1 wherein said molten solvent drop is iron.

4. A process for growing a single crystal filament as set forth in claim 1 wherein said substrate has a seed crystal of silicon carbide thereon upon which said filament crystal grows.

5. A process for growing a single crystal filament as set forth in claim 1 wherein said selected element is silicon carbide.

6. A process for growing a single crystal filament as set forth in claim 5, wherein said vapor containing said molten solvent is the decomposed products of iron pentacarbonyl.

7. A process for growing a single crystal filament as set forth in claim 1 wherein said substrate has a seed crystal of silicon thereon upon which said filament crystal grows.

8. A process for growing a single crystal filament as set forth in claim 7, wherein said selected element is silicon.

9. A process for growing a single crystal filament as set forth in claim 8, wherein said vapor containing molten solvent is the decomposed products of iron pentacarbonyl.

forth 

2. A process as set forth in claim 1 wherein said temperature is maintained by translating said continuous filament crystal in a direction opposite said continuous filament crystal growth and at the rate of growth of said continuous filament crystal to thereby cause said molten drop to maintain a predetermined temperature gradient therein.
 3. A process for growing a single crystal filament as set forth in claim 1 wherein said molten solvent drop is iron.
 4. A process for growing a single crystal filament as set forth in claim 1 wherein said substrate has a seed crystal of silicon carbide thereon upon which said filament crystal grows.
 5. A process for growing a single crystal filament as set forth in claim 1 wherein said selected element is silicon carbide.
 6. A process for growing a single crystal filament as set forth in claim 5, wherein said vapor containing said molten solvent is the decomposed products of iron pentacarbonyl.
 7. A process for growing a single crystal filament as set forth in claim 1 wherein said substrate has a seed crystal of silicon thereon upon which said filament crystal grows.
 8. A process for growing a single crystal filament as set forth in claim 7, wherein said selected element is silicon.
 9. A process for growing a single crystal filament as set forth in claim 8, wherein said vapor containing molten solvent is the decomposed products of iron pentacarbonyl. 